The present invention relates generally to the reduction of electromagnetic emissions from electronic equipment, and particularly to a face plate which will eliminate compromising emanations from the face of a video display device, such as a cathode ray tube display (CRT).
A compromising emanation is defined as electromagnetic energy which, while unintentionally emitted from electronic equipment processing information, particularly classified information, has some characteristic that makes it possible to intercept and recover the information processed on the equipment. Generally, a compromising emanation is a detectable signal containing information that the user of electronic data processing equipment does not wish known outside of his organization.
Previous prior art techniques to eliminate emanations from the face of a CRT or other video display device have included the use of an electromagnetic emission shield to cover the CRT face. For example, emission shields made of fine mesh woven of stainless steel wire of approximately 0.001" diameter are commonly used. The woven mesh is cut to size, silver plated to enhance electrical conductivity, and chemically blackened to mask its presence. The mesh can be used alone by stretching it over the CRT face, or as part of a face plate assembly. In such an assembly, the mesh is placed between the sheets of glass with a bonding agent, and through the use of heat, pressure, etc., the sheets of glass are laminated together in a single assembly which is then placed on front of the CRT. The mesh, when grounded to a metallic housing or other suitable structure, prevents the passage of the compromising emanations.
Typically, the mesh is either bonded to the edge of the assembly by the use of a conductive buss around the periphery of the assembly, or the mesh is allowed to extend beyond the edge of the glass face plate assembly. The mesh or face plate assembly is grounded to the surrounding structure by fastening a ground onto the edge buss, or the extended mesh. Using the extended mesh presents the simpler task since the flexibility of the wire mesh allows the assembly to be positioned at the optimum location. When using the conductive buss, the assembly must be precisely located in order for an electrically conductive seal to be made all around the part. In addition, the design of the assembly must be much more precise than with the extended mesh, and must be specific to a particular CRT display.
A second technique used to control compromising emanations from a CRT is the use of a transparent, electrically conductive coating on a sheet of glass or plastic. The coating is applied to one side of a sheet of transparent material. This can then be used as is, or since the coatings are thin and fragile, and can be easily scratched, protection for the conductive coating can be provided by adding another sheet of the transparent material, bonding the sheets together with a transparent bonding material such than the conductive coating is between the two sheets. Electrical contact is made with the conductive coating by forming a conductive buss around the periphery of the assembly.
As with the mesh face plate discussed above, the assembly containing the conductive coating material must be electrically bonded to a metallic housing or other suitable structure in order to ensure a compromising emanation tight assembly for the CRT and associated electronics. As discussed above, the assembly with the conductive buss must be precisely located and designed for a particular CRT display to insure electrical contact around the entire periphery of the plate assembly face.
However, the prior art techniques explained above suffer from a number of problems. The first is the use of a wire mesh face plate with a color CRT. The physical orientation of the wire mesh on the face of the CRT vs. the dot-matrix screen pattern inside of the CRT can cause wavy interference patterns, called Moire patterns, in the displayed image. These patterns are a nuisance, creating an unfavorable opinion of the product; further, the patterns can cause misinterpretation of the displayed information. A similar problem occurs when a wire mesh screen is used with a monochrome display. The wire mesh can interfere with the displayed image, reducing clarity and producing an unfocused condition. This problem is especially severe with images such as small text. A third problem area with the mesh face plates is that of mesh quality and cost. Uneven weave in the mesh will create dark streaks across the displayed image. High reject rates of mesh stock because of uneven weaving, plating, blackening, etc., increase the final product cost. The number of suppliers of high quality mesh is limited, also increasing product cost.
The use of face plates with a conductive coating eliminates the mesh problems and has minimal impact on the displayed image (colors darken slightly, contrast is increased). The cost is roughly equal to that of mesh; however, there is usually difficulty in the mounting of a coated face plate between the CRT and the front bezel. There is limited space around the CRT/bezel interface in which to add a feature which will make a positive electrical contact to the conductive buss which runs around the outside of the face plate assembly.